In a conventional charging step included in electrophotographic processes using an electrophotographic photosensitive member, in most cases a high voltage (DC voltage of about 5-8 KV) is applied to a metal wire to generate a corona, which is used for the charging. In this method, however, a corona discharge product such as ozone and NOx is generated along with the generation of the corona. Such a corona discharge product deteriorates the photosensitive member surface and may cause deterioration of image quality such as image blurring or fading or the presence of black streaks across the copy sheets. Further, ozone contamination may be harmful to humans if released in relatively relatively large quantities. In addition, a photosensitive member that contains an organic photoconductive material is susceptible to deterioration by the corona products.
Also, as the power source, the current directed toward the photosensitive member is only about 5 to 30% thereof. Most of the power flows to the shielding plate. Thus, the efficiency of the charging means is low.
For overcoming or minimizing such drawbacks, methods of charging have been developed using a direct charging member for charging the photosensitive member. For example, U.S. Pat. No. 5,017,965 to Hashimoto et al., uses a charging member having a surface layer which comprises a polyurethane resin. Another approach, European Patent Application 0 606 907 A1, uses a charging roller having an elastic layer comprising epichlorohydrin rubber, and a surface layer thereover comprising a fluorine containing bridged copolymer.
These and other known charging members are used for contact charging a charge-receiving member (photoconductive member) through steps of applying a voltage to the charging member and disposing the charging member being in contact with the charge-receiving member. Such bias charging members require a resistivity of the outer layer within a desired range. Specifically, materials with resistivities which are too low will cause shorting and/or unacceptably high current flow to the photoconductor. Materials with too high resistivities will require unacceptably high voltages. Other problems which can result if the resistivity is not within the required range include nonconformance at the contact nip, poor toner releasing properties and generation of contaminant during charging. These adverse affects can also result in bias charging members having non-uniform resistivity across the length of the contact member. It is usually the situation that most of the charge is associated at or near the center of the charge member. The charge seems to decrease at points farther away from the center of the charge member. Other problems include resistivity that is susceptible to changes in temperature, relative humidity, running time, and leaching out of contamination to photoconductors.
Due to its contact, the direct charging apparatus also causes more wear and tear to itself, imaging members and any other components with which it comes in contact. Failure modes in a bias charge roller (BCR) show up in prints such as dark streaks, and white and dark spots, which are associated with surface damages on BCR. These defects are usually derived from degradation or debris build-up on the BCR surface along the circumference, i.e. the process direction. The degradations can be scratches, abrasion, or pothole-like damages to the BCR surface. Another known deficiency is toner filming on the BCR surface that can also show up as print streaks. All these failures will reduce BCR life and therefore limit usage life.